1. Field of the Invention
The present invention relates to a constant-voltage automatic charging strobe circuit used as the strobe unit circuit for easy-use cameras such as disposable cameras, compact cameras, or digital still cameras.
2. Description of the Related Art
In general, most of the compact cameras on the market are equipped with an automatic exposure function and use a battery with nominal voltage of 3 volts or greater.
In the circuit block of the camera, the automatic exposure circuit is generally constituted so as to be within the shutter block and to operate at timing that is shifted from the timing of the strobe circuit block in a series of camera operations. The power source may be either a 3-volt power source without further enhancement, or converted once to 4 volts or 5 volts by a DC/DC converter.
Also, for a nominal battery voltage of 1.5 volts it is necessary to provide a solenoid of corresponding size in order to operate the aperture blades of the camera exposure mechanism, for example. Because of the high current flowing to the solenoid, the number of parts becomes large and the time until battery replacement also becomes shortened in practical application.
Next, when considering a constant-voltage strobe circuit, the basic operating manners are all the same, namely, detect the charging voltage of a main capacitor, turn off a power switching transistor (oscillation control transistor) connected to the primary side of a transformer when an established voltage is reached, and effect oscillation and stop charging. To realize the means for detecting the abovementioned charging voltage, there are the method of using a voltage detecting element such as a Zener diode, a varistor or neon lamp, and the method of preparing an intermediate tap on the secondary winding side of the transformer, using the output voltage generated thereby and turning off the oscillation control transistor. Furthermore, combinations of these methods may be used. However, circuits to which these methods are applied all have complex structures and become expensive.
Also, the method for automatically controlling the quantity of light emitted by the strobe uses a parallel control circuit or serial control circuit for controlling the quantity of light using a thyristor or quench tube. Furthermore, in recent years, the method for controlling the photocurrent flowing directly to a xenon tube by using IGBT as a new power device is becoming mainstream.
Using these circuits makes it possible to attain a continuously stable quantity of light corresponding to changes in distance. However, these have a large number of circuit elements and the main device is expensive. As a result, the low price that is one of the merits of easy-use cameras does not materialize. In view of this situation, many types of the easy-use cameras that are currently available use manually controlled circuits having no light-quantity correction circuits.
In this way, in conventional compact cameras, it is necessary to select batteries with high nominal voltages or to use complex and large-scale circuitry to attain the high driving current in order to drive the load (solenoid, etc.) for operating the aperture blades and so forth in the automatic exposure mechanism. The problem is that with this type of load, cost increases are unavoidable and battery life has to become short.
Also, in conventional compact cameras, an automatic oscillation stopping circuit, for turning off the oscillation control transistor upon detecting a set voltage and causing the oscillation action to stop, is generally realized by using a voltage detecting element such as a Zener diode, or by using a more complex circuit with an intermediate tap connected to the secondary winding side of a transformer, using the output voltage generated thereby, and turning off the oscillation control transistor. As a result, the problem is that cost increases for the camera as a whole are brought about by the cost increases from this circuitry.
Also, conventional cameras include those having a circuit for automatically controlling the quantity of light emitted by the strobe that controls the quantity of light emitted by the strobe circuit using a thyristor or quench tube. Because these circuits include many circuit elements and the price of the main device is also high, it is not practical to include this in an inexpensive camera and these types of inexpensive cameras therefore do not have a function for preventing overexposure when taking strobe pictures at close range.